Epicuticular wax

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Epicuticular wax

In Botany the Plant Cuticle is covered by epicuticular wax mainly consisting of straight-chain aliphatic hydrocarbons with a variety of substituted groups. Common examples are paraffins in leaves of peas and cabbages, alkyl esters in leaves of Carnauba palm and banana, the asymmetrical secondary alcohol 10-nonacosanol in most conifers, many of the Ranunculaceae, Papaveraceae and Rosaceae and some mosses, symmetrical secondary alcohols in Brassicaceae including Arabidopsis thaliana, primary alcohols in most grasses, Eucalypts and Legumes among many other plant groups, β-diketones in many grasses and Eucalypts, aldehydes in young beech leaves, sugar cane culms and lemon fruit and triterpenes in fruit waxes of apple, plum and grape (Baker 1982; Holloway and Jeffree 2005).

These compounds are mostly soluble in organic solvents such as chloroform and hexane, making them accessible for chemical analysis, but in some species esterification of acids and alcohols or polymerization of aldehydes may give rise to insoluble compounds. Epicuticular wax can now also be isolated by mechanical methods (Ensikat, Neinhuis and Barthlott, 2000) which distinguish the epicuticular wax outside the Plant Cuticle from the cuticular wax embedded in the cuticle polymer. The two are chemically distinct (Jetter, Schäffer, and Riederer 2000), although the mechanism which segregates the molecular species is unknown.

Epicuticular wax forms crystalline projections from the plant surface, which enhance their water repllency (Holloway, 1969; Barthlott and Neinhuis 1997). The shapes of the crystals are dependent on the dominant wax compounds. Asymmetrical secondary alcohols and β-diketones form hollow wax nanotubes, while primary alcohols and symmetrical secondary alcohols from plates (Hallam, 1967; Jeffree, Baker and Holloway 1975). Although these have been observed using the Transmission Electron Microscope (Juniper and Bradley 1958; Hallam 1967) and Scanning Electron Microscope (Jeffree 2006 and references therein) the process of growth of the crystals had never been observed until the Atomic force microscope studies of Koch and coworkers (2004, 2005). These studies show that the crystals grow by extension from their tips, raising interesting questions about the mechanism of transport of the molecules.

Epicuticular Wax References

Baker, E. A. (1982) Chemistry and morphology of plant epicuticular waxes, in The Plant Cuticle, (eds D. J. Cutler, K. L. Alvin, and C. E. Price), Academic Press, London, pp. 139-165.
Hallam, N.D. (1967) An electron microscope study of the leaf waxes of the genus Eucalyptus L'Heritier, PhD thesis, University of Melbourne.
Holloway, P. J. and Jeffree, C. E. (2005) Epicuticular waxes, Encyclopedia of Applied Plant Sciences, 3, pp. 1190-1204.
Riederer, M. & Müller, C., eds (2006) Biology of the Plant Cuticle. Blackwell Publishing. [link]
Barthlott, W. and Neinhuis, C. (1997) Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta 202, 1–8.
Eigenbrode, S.D. (1996) Plant surface waxes and insect behaviour, in Plant Cuticles: an integrated functional approach, (ed G. Kerstiens), Bios Scientific Publishers, Oxford, pp. 201-221.
Ensikat, H. J., Neinhuis, C., and Barthlott, W. (2000) Direct access to plant epicuticular wax crystals by a new mechanical isolation method, International Journal of Plant Sciences, 161, 143-148.
Holloway, P. J. (1969) The effects of superficial wax on leaf wettability, Annals of Applied Biology, 63, 145-153.
Jeffree, C.E. (2006)The fine structure of the Plant Cuticle. Chapter 2 In: Riederer, M. & Müller, C., eds (2006) Biology of the Plant Cuticle. Blackwell Publishing. pps 11-125.''
Jeffree, C. E., Baker, E. A., and Holloway, P. J. (1975) Ultrastructure and recrystallisation of plant epicuticular waxes, New Phytologist, 75, 539-549.
Jetter, R., Schäffer, S., and Riederer, M. (2000) Leaf cuticular waxes are arranged in chemically and mechanically distinct layers: evidence from Prunus laurocerasus L., Plant, Cell and Environment, 23, 619-628.
Juniper, B. E. and Bradley, D. E. (1958) The carbon replica technique in the study of the ultrastructure of leaf surfaces, Journal of Ultrastructure Research, 2, 16-27.
Koch, K., Neinhuis, C., Ensikat, H. J., and Barthlott, W. (2004) Self assembly of epicuticular waxes on living plant surfaces imaged by atomic force microscopy (AFM), Journal of Experimental Botany, 55, 711-718.
Koch, K., Barthlott, W., Koch, S., Hommes, A., Wandelt, K., Mamdouh, H., De-Feyter, S. and Broekmann P. Structural analysis of wheat wax (Triticum aestivum, c.v. 'Naturastar' L.): from the molecular level to three dimensional crystals, Planta, 223, 258-270

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